1897.] on Some Curiosities of Vision. 361 



I propose now to say a few words about a curious phenomenon 

 of vision wbicli occupied my attention towards the end of hist year.* 



Eather more than two years ago, Mr. C. E. Benham brought out 

 a pretty little toy which he called the Artificial Spectrum Top. It 

 consists of a cardboard disc, one half of which is painted black, while 

 on the other half are drawn four successive groups of concentric 

 black lines at different distances from the centre. When the disc 

 rotates rather slowly each group of black lines generally appears to 

 assume a different colour, the nature of which depends upon the speed 

 of the rotation and the intensity and quality of the light. Under 

 the best conditions the inner and outer groups of lines become 

 bright red and dark blue ; at the same time the intermediate groups 

 also appear tinted, but the hues which they assume are rather un- 

 certain and difficult to specify. By far the most striking of the 

 colours exhibited by the top is the red, and next to that the blue ; 

 this latter, however, is sometimes described as bluish-green. [The 

 top was exhibited as a lantern slide.] 



My recent experiments seem to indicate pretty clearly the cause 

 of the remarkable bright red colour and also that of the blue. The 

 more feeble tints of the two intermediate groups of lines perhaps 

 result from similar causes in a modified form, but these I have not 

 yet investigated. 



In the red colour we have another striking example of an ex- 

 ceedingly common phenomenon which is habitually disregarded ; 

 indeed, I can find no record of its ever having been noticed at all. 

 The fact is, that whenever a bright image is suddenly formed upon 

 the retina after a period of comparative darkness, this image appears 

 for a short time to be surrounded by a narrow coloured border, the 

 colour under ordinary conditions of illumination being red. If the 

 light is very strong the transient border is greenish-blue. Sometimes 

 both red and blue borders appear together, the blue being inside the 

 red.t The colour generally seen is, however, red, and it is most 

 conspicuous with good lamp-light. 



This observation was first made in the following manner. A 

 blackened zinc plate with a small round hole in it is fixed over a 

 larger hole in a wooden board ; the hole in the zinc is covered with 

 a piece of thin white writing paper. Thus we are furnished with a 

 sharply defined translucent disc which is surrounded by a perfectly 

 opaque substance. An arrangement is made for covering the trans- 

 lucent disc with a shutter which can be opened very rapidly by 

 means of a strong spring. If this apparatus is held between the 

 eyes and a lamp, and the translucent disc is suddenly disclosed by 

 working the shutter, the disc appears for a short time to be sur- 

 rounded by a narrow red border. The width of the border is perhaps 



* Proc. Koy. Soc. vol. Ix. p. 370 (1896). 



t I have recently shown that the greenish-blue border is simply the " negative 

 after-image " of the red one. — April 24th. 



